Field of the Invention
The invention relates to a substrate of semiconductor material used to produce semiconductor devices including optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs), and electronic devices such as transistors. More specifically, the invention provides substrates of group III nitride such as gallium nitride. The invention also provides methods of making these substrates.
Description of the Existing Technology
This document refers to several publications and patents as indicated with numbers within brackets, e.g., [x]. Following is a list of these publications and patents:                [1] Philippe Spiberg, et al., U.S. Patent Pub. No. 2011/0108954A1.        [2] R. Dwilinski, et al., U.S. Pat. No. 7,132,730B2.        [3] H. Osada, et al., U.S. Patent Pub. No. 2012/0034763 A1        
Each of the references listed in this document is incorporated by reference in its entirety as if put forth in full herein, and particularly with respect to their description of methods of making and using group III nitride substrates.
Gallium nitride (GaN) and its related group III nitride alloys are the key material for various optoelectronic and electronic devices such as LEDs, LDs, microwave power transistors, and solar-blind photo detectors. Currently LEDs are widely used in displays, indicators, general illuminations, and LDs are used in data storage disk drives. Recently, researchers have demonstrated GaN with m-plane (nonpolar), a-plane (nonpolar), angled m-plane (semipolar), or angled a-plane (semipolar) shows higher indium incorporation when solid solution of InGaN is grown. Higher indium content is required to fabricate light emitting devices with longer wavelength such as green, amber and even red.
To obtain such nonpolar and semipolar substrates, there are a few approaches. One is to use non-c-plane heterogeneous substrate such as r-plane sapphire, m-plane silicon carbide to grow GaN by vapor phase epitaxy [1]. Since large diameter (>2″) wafers of these materials are commercially available, it is relatively easy to obtain large-area nonpolar/semipolar GaN on such substrates. For example, a-plane GaN can be grown on r-plane sapphire and m-plane GaN can be grown on m-plane SiC. Similarly, some researchers proposed to use a patterned c-plane sapphire substrate to obtain non-polar/semipolar GaN. This approach also provides relatively large-area nonpolar/semipolar GaN layers. However, these approaches inevitably introduce basal plane stacking fault, which propagates parallel to c-plane of GaN. Typical density of the stacking faults are 105 cm−1, which means the average spacing of the stacking faults is 0.1 microns. Scientists have proven that the stacking faults become non-radiative recombination center, thus optical devices fabricated on such material does not have high efficiency.
To obtain high-quality nonpolar/semipolar GaN substrates with less stacking faults, people grow bulk crystal of GaN along c-axis direction and slice the crystal along the desired orientation such as m-plane and semipolar planes [2]. Since growth along c-axis does not introduce stacking faults, people can obtain practically stacking fault-free crystal in this approach. However, when m-plane wafers are sliced from a bulk GaN crystal, the slicing direction is perpendicular to the c-plane, thus the size of the sliced wafer is limited by the thickness of the bulk GaN crystal. This situation is similar for other nonpolar/semipolar orientations. Thus, this approach typically provides long strips of wafers. For example, if 2″ diameter×5 mm thick bulk GaN crystal is sliced to obtain m-plane wafers, the largest piece one can obtain is a 2″×5 mm rectangular strip. Although this strip contains negligible amount of stacking faults, the shape and size is not favorable for commercial application.
One method to obtain large-area nonpolar/semipolar wafer using the strips explained above is to tile these strips and grow GaN on the exposed nonpolar/semipolar surface to fuse these strips together [3]. However, this method has a problem of growing polycrystalline along the gap between two strips, therefore, crystal quality is typically poor.